Very useful patch!

The only thing I'm unsure of is the three-second delay. It might be a bit too long if some annoying applicaiton suddenly starts blaring sound and the option to mute it easily only becomes available after three seconds of torture.
I think two or maybe even one second might already be enough to keep the icon from blinking.

Note that the mute option is immediately available in the context menu and stays there even if the playback has finished (if the application keeps the stream open, e.g. a player that is just paused). The three seconds delay only applies to the icon but it could surely be reduced, given the duration of the fade-in for the icon is also 1s right now.

Love this (well I suggested it :) and would be a nice first for Plasma Desktop. Code actually looks pretty good.

+1 really cool!

About vert panels, maybe if the button becomes big enough it should show as a corner badge?

maybe if the button becomes big enough it should show as a corner badge?

I tried, using a ShaderEffect like for the badge but I couldn't get it to work well or look great, especially since it needs to completely change for when there is a label (where it looks beautiful as seen in the screenshot).

How should we proceed here? Easiest option would be to not show the indicator if the button is too small but that'll surely keep the feature from icon tasks. :/

Could this be a solution? The icon needs to be reduced a little bit in size maybe when showing the indicator.

I like the idea but depending on the icon, especially non-round ones, might look terrible.

and when I make a new emblem icon for audio on/off this would have an background circle and inside the mute/volume on icon like we have it for owncloud, ...

Last try: It's the speaker symbol without the speaker. Should be easy to make by just cutting away the speaker. Plus vertically centered to the app icon.